Particulate material is prepared by mixing binder such as wax into particles of ceramic or metal. The particulate material thus prepared is compressed in a mold of a pressing machine, and then sintered in a furnace to be used as a carbide tip, a precision machinery component and so on.
According to a conventional method for compressing the particulate material using a conventional pressing machine, an upper punch and a lower punch of the pressing machine are reciprocated slowly by a crank mechanism or a hydraulic mechanism. In addition, friction between the particulate materials is rather high. Therefore, the particulate material cannot be compressed densely by a conventional pressing machine, and density distribution of the particulate materials thus compressed by the conventional pressing machine is not sufficiently homogeneous.
One example of a conventional powder pressing machine is disclosed in Japanese Patent Laid-Open No. 2004-174596. According to the teachings of Japanese Patent Laid-Open No. 2004-174596, a punch is attached individually to an upper and lower rams through a lamination type piezoelectric element, and powder filled in a metal mold is compressed smoothly into a desired shape by applying impact force intermittently. Therefore, the powder pressing machine taught by Japanese Patent Laid-Open No. 2004-174596 is capable of resolving the above-explained disadvantages.
The impact-type powder pressing machine taught by Japanese Patent Laid-Open No. 2004-174596 is shown in FIG. 11. As shown in FIG. 11, the powder pressing machine taught by Japanese Patent Laid-Open No. 2004-174596 comprises: a frame 1; an intermediate frame 11; an upper ram 2; a ball screw 21 for reciprocating the upper ram 2; a lamination type piezoelectric element 23; an upper punch 3 attached to the upper ram 2 through the piezoelectric element 23; a die 4 fixed to the intermediate frame 11; a lower ram 5; a ball screw 51 for reciprocating the lower ram 5; a lamination type piezoelectric element 52; and a lower punch 6.
For example, a Piezo-Electric Transducer (abbreviated as PZT) is known as the piezoelectric element in the prior art. Specifically, PZT is a ceramic element which is deformed instantaneously by applying driving voltage thereto.
However, as explained in paragraph [0030] of Japanese Patent Laid-Open No. 2004-174596, a range of deformation of the piezoelectric element is not very wide e.g., within several μm and several 10 μm. Therefore, according to the powder pressing machine taught by Japanese Patent Laid-Open No. 2004-174596, a plurality of piezoelectric elements have to be laminated to form the lamination type piezoelectric element. Further, the lamination type piezoelectric element cannot function effectively if a spring back amount of the powder material (i.e., a difference between thicknesses thereof when compressed and after compressed) is larger than a deformation amount thereof.
Basically, the impact force is not oriented to the specific direction. Therefore, the powder pressing machine taught by Japanese Patent Laid-Open No. 2004-174596 has to be improved to concentrate the impact force to vertical direction.
In addition to the above-explained disadvantages, the inventors of the present invention have found a fact that the impact force cannot be transmitted effectively in the powder material and voids would remain in the powder material, without applying predetermined pressure to the powder material in advance of applying the impact force thereto. Therefore, according to the teachings of Japanese Patent Laid-Open No. 2004-174596, the impact force cannot propagate entirely into the powder material to compress the powder material homogeneously.